Ventilator-induced lung injury (VILI) is a common cause of the morbidity and mortality associated with excessive lung cell stretch produced by mechanical ventilation. High permeability pulmonary edema is a cardinal feature of VILI, however, the mechanisms involved are unclear. These changes likely involve direct damage to endothelial cells (EC) or "stress failure", as well as subtle events involving EC signaling pathways which modulate by an actin-based system that effects cell contraction and stress fiber formation. We hypothesize that endothelial cells exposed to excessive cyclic stretch undergo phenotypic changes that result in increased contractile protein expression resulting in enhanced agonist-driven contractility and permeability. Utilizing an in vitro model, human pulmonary artery endothelial cells (HPAEC) and human microvascular endothelial cells (HMEC) will be exposed to cyclic stretch (5% and 18% radial elongation). In specific aim #1, we will characterize the time- and strain-dependent effect of stretch on macro and micro lung EC. In specific aim #2, using Affymetrix cDNA microarrays, we will determine altered contractile protein gene expression patterns modified by cyclic stretch. EC contraction is often regulated by EC myosin light chain kinase (MLCK), an enzyme first cloned by the sponsor's laboratory that drives myosin light chain phosphorylation, a precursor of actomyosin contraction. In specific aim #3, the role of MLCK in EC barrier dysfunction after stretch will be investigated with measures of MLCK activity and elaboration of the effects of pharmacologic inhibitors as well as and overexpression of MLCK mutant constructs. These studies, which define EC biochemical and genomic response to cyclic stretch, may result in new therapeutic strategies that target VILI.